1 86 BIOLOGY OF SPECIALIZED GROUPS 



bizarre shapes, which are usually known as bacteroids (Fig. 

 44). These legume bacteria make their way into the roots 

 through the root hairs and induce the formation of tuber- 

 cles, or nodules. In these the bacteria live and rapidly 

 multiply, and it is here that they take the elemental nitro- 

 gen of the air and fix it in a form available for plant use. 

 It has been abundantly shown that the power of these 

 leguminous plants to make use of the nitrogen of the air 

 depends on the activity of these microorganisms. If these 

 legumes are sown in sterile soil and protected from inocula- 

 tion, they will cease to grow as soon as the nitrogen of the 

 seed has disappeared; whereas, other plants under exactly 

 the same condition, except that they have been infected with 

 pure cultures or soil washings, continue to grow. These 

 legume bacteria are of extreme importance in agricultural 

 pursuits, and will be referred to again in a later chapter. 

 Other bacteria, found in the soil, are able to fix nitrogen when 

 growing independently of legumes or plants. 



Sulphur Bacteria. Many bacteria (putrefactive) are 

 able to produce sulphuretted hydrogen, but the sulphur bac- 

 teria proper take this hydrogen sulphide that has been pro- 

 duced by another group and use it as food, oxidizing into 

 sulphuric acid (Fig. 45). These sulphur bacteria belong to 

 two groups, one of which is colorless and the other contains 

 a coloring matter. Thiothrix may be taken as an example of 

 the filamentous forms of the colorless group. The Thiothrix 

 is composed of a thread of cells which is usually attached to 



